The invention relates to a tread pattern for a tread intended for a tire for heavy vehicles; it also relates to a mold making it possible to obtain such a tread pattern by molding.
In order to maintain, or even improve, the performance of a tread pattern of a tire comprising elements in relief under conditions of travel on slippery ground (such as, for example, ground covered with water), it is known to make on said elements a plurality of cutouts in the form of grooves and/or incisions (an incision having a width less than the width of a groove, and generally the width of an incision is less than 3 mm). These cutouts in total cross the width of the rib to interconnect the grooves defining said ribs.
It is also known that the performance on slippery ground is improved when the number of cutouts is large; however, an excessively large number of cutouts results in a great loss of rigidity of the elements in relief, which results in a general loss of performance, even on non-slippery ground. In fact, this loss of rigidity causes movements of greater amplitude for the points of the tread pattern which are in contact with the ground, which causes, among other disadvantages, so-called irregular wear in that they are localized (in particular on the ridges of the ribs) and no longer uniformly distributed over the entire running surface of the tread. This irregular wear significantly reduces the useful life of the tire provided with said tread pattern.
It is also known that the distributions of contact pressure between each element in relief of a tread pattern and the ground have a direct influence on the regularity of wear of said tread. For example, in the case of a tread pattern provided with a plurality of ribs defined by grooves of substantially longitudinal orientation (corresponding to the circumferential direction on the tire), it is known to mold grooves which, when viewed in cross-section, have widths which increase as the tread becomes worn.
By definition, the walls of the elements in relief defined by such grooves are said to be undercut relative to the running surface of the tread.
This arrangement makes it possible to make the ribs flexible in the vicinity of their ridges (defined as being the intersections of the lateral walls of each rib with its wall surface intended to come into contact with the ground) and thus to reduce the contact pressures of each rib with the ground close to said ridges. This reduction in pressure is more particularly substantial when the tread is new, that is to say, when the thickness of the tread is at a maximum.
However, if said latter tread pattern is provided with a plurality of incisions to increase the performance of adhesion on slippery ground, then the combination of a low pressure in the vicinity of the ridges of the ribs with movements of great amplitude linked to the presence of said incisions results in very pronounced irregular wear of these ribs in the vicinity of their ridges.
One known method of avoiding this irregular use is to make a plurality of incisions opening solely on to the surface wall of the ribs without cutting into the lateral walls and the ridges of the ribs; however, this arrangement results in noise problems which are linked to air being trapped within said incisions when they pass into contact with the ground during travel.
One object of the invention is to propose a tread for a tire, the tread pattern of which comprises a plurality of elements in relief defined by at least two grooves, and at least two of the lateral walls of which are undercut, some of said elements in relief being provided with at least one incision opening on to the running surface of the tread when new, said tread not having the disadvantages which have been discussed above.
There is proposed a tread for a tire which is provided with a tread pattern comprising a plurality of elements in relief defined by grooves, each element in relief being provided with a surface wall forming part of the running surface and intended to come into contact with the ground during travel, and at least two lateral walls which are undercut relative to the surface wall, the intersection of each of the lateral walls with the surface wall forming a ridge.
The tread according to the invention is characterized in that:
at least one element in relief is provided with at least one incision opening on to the surface wall thereof and on to at least one of its lateral walls, and
in that the incision(s) do(es) not cut into the ridge common to said surface wall and to said lateral wall at least between a first depth H1 and a second depth H2, H2 being greater than Hi, these depths H1 and H2 being measured perpendicular to the surface wall of the element and corresponding to different levels of wear of the tread.
At least two of the lateral walls of one and the same element in relief are undercut, that is to say that they are inclined such that the width of the surface wall of said element decreases as the tread becomes worn.
The tread pattern according to the invention makes it possible to improve the performance of the tread pattern on ground which has been made slippery, owing to a number of incisions which may be large, without creating a problem of noise or of irregular wear on dry ground. Each element in relief of the tread pattern according to the invention may comprise a plurality of incisions which open both onto the contact surface of the element with the ground and into at least one of the grooves defining the element in relief to permit the evacuation to at least one of said grooves of the fluids entering the incision during travel.
It should be noted that as a general rule the maximum depth of the incisions is virtually equal to the depth of the grooves defining the elements in relief to benefit from a large number of ridges during the entire period of use of the tread; however, it is of course possible to provide different depths.
Preferably, the depth H1 is low compared with the depth of the incision in question, so as to retain sufficient rigidity for the element in relief, in particular when said element is subjected to forces of contact with the ground which are directed parallel to the ground and in a direction perpendicular to the average orientation of the incision; xe2x80x9clowxe2x80x9d is understood to mean that H1 is at most equal to 10% of the deepest of the cutouts (grooves and incisions included).
Each incision has on the running surface a geometric trace having an average orientation defined as being the angle formed with the longitudinal direction of the tread by the straight-line segment joining the points of said trace which are farthest from each other.
The tread pattern according to the invention has the advantage of perceptibly reducing the travelling noise on dry ground created by the presence of incisions due to the fact that these incisions open into at least one groove and can thus evacuate the air which they contain as they pass into contact with the ground.
This tread pattern, despite the large number of incisions, has the advantage of modifying relatively little the distribution of the contact pressures of each element on the ground, in particular close to the ridges, while alleviating the reduction in rigidity during the first part of use of the tread (that is to say, up to about a level of wear corresponding to half the thickness of said tread).
A good compromise between the maintenance of sufficient rigidity for an element in relief and optimum evacuation of the fluids contained in the incisions of said element in relief towards the adjacent grooves is achieved once the depth H2 is between 40% and 90% of the depth of said incisions.
Preferably, the incisions which open both on to the surface wall and on to at least one lateral wall of an element in relief do not cut into any of the ridges of said element when new (in this case: H1=0); the object of this arrangement is not to reduce the rigidity of the tread in the vicinity of the surface wall when new.
One particularly advantageous variant consists in distributing over one and the same element in relief of the tread pattern a plurality of incisions opening on to the surface wall of the tread when new, such that these incisions open alternately on one and then on the other lateral wall of said element without, for example, cutting into the ridges of this element when new. Thus after partial wear of the element in relief, the incisions opening on to the surface wall of said element only cut into a single ridge of the element, which makes it possible to maintain a balance of rigidities of the element in relief between one side of the element in relief and the other, avoiding sudden weakening of said rigidities.
To produce a tread pattern comprising at least one element in relief provided with at least one cutout opening on to the surface wall and on to part of at least one lateral wall of this element, it is clear that existing tire molds are not suitable, the main difficulty lying in demolding the molding elements from the mold without causing breaking of the ridges of the elements in relief. xe2x80x9cDemoldingxe2x80x9d is understood to mean the extraction of these molding elements from the material constituting the tread after molding and vulcanization.
One known possibility consists in using a filler material for molding each incision before removing said material after molding; however, the difficulty of working and the high cost of such a method from an industrial point of view should be noted.
One object of the invention is to produce a mold for molding a rubber tread provided with such a tread pattern directly during the molding and vulcanization phase of a tire without necessarily having to mold the tread in a mold which simultaneously molds the inner and outer faces of this tread. This embodiment then requires an assembly stage with a tire blank manufactured separately. This is of course particularly costly, and is not without problems of strength of the bond between a prevulcanized tread and a tire blank, whether or not prevulcanized.
The mold according to the invention comprises a plurality of mold parts each having their own direction of molding/demolding. Each mold part has a molding surface intended to mold part of the running surface of a tread. Furthermore, each mold part is provided with molding elements which project over its molding surface. First molding elements, which are intended to mold grooves defining elements in relief of the tread pattern, comprise lateral faces and end faces limiting said lateral faces; these lateral faces are intended to mold the main lateral walls of the elements in relief on the tread. Second molding elements are intended to mold cutouts in the elements in relief; these molding elements also comprise lateral faces and end faces limiting said lateral faces. The lateral faces mold the lateral walls of cutouts of average orientation different from the average orientation of the grooves molded by the first molding elements.
The mold according to the invention is characterized in that:
at least one lateral face of a molding element and one end face of another molding element of the mold part cooperate so as to be in contact and to define, at least in part, an orifice extending between a depth H1 and a depth H2 (H2 being greater than H1) to allow the rubber to pass during the molding of a tread, said depths H1 and H2 being measured from and perpendicular to the molding surface of the mold part, and
one of said molding elements in contact with the other molding element is mounted to be mobile relative to the mold part and to this other molding element in a direction close or identical to the direction of molding/demolding of the mold part, so that, during a first phase of the demolding which consists in moving the mold part in its demolding direction, the resultant forces exerted by the rubber molded in each orifice on the mobile molding element force said element to remain in the tread and thus no longer to be in contact with the other molding element so as to permit, in a second phase of the demolding, the extraction of the mobile molding element from the tread and thus to finish the demolding without breaking the rubber molded in each orifice.
The expression xe2x80x9cto define, at least in part, an orificexe2x80x9d indicates that the orifice in question is entirely defined by walls of two molding elements (this configuration corresponds, for example, to the variant shown in FIG. 5), or that it is defined both by walls of two molding elements and the molding surface of the mold part provided with said molding elements (the latter configuration corresponds, for example, to the variant shown in FIGS. 3A, 3B, 3C).
A mold according to the invention offers the advantage of making it possible industrially to mold a tread pattern of a tread having a pattern at least one element in relief of which comprises two of its lateral walls undercut and is provided with at least one incision opening on to the surface wall and on to at least one of the lateral walls of said element at least between a first depth H1 and a second depth H2, H2 being strictly greater than H1.